The Sling Materials and What Each One Cannot Tolerate

Why this matters

Sling selection gets taught as an arithmetic problem, and the arithmetic is the easy half. Work the load, the share, the angle, the hitch and the bend, and you get a required rated capacity. That number tells you what size. It does not tell you what the sling should be made of, and it never will, because the thing that kills a sling early is almost never load.

The environment is what kills it, and each family fails to a different environment. The family that shrugs off the acid in one bay is the family that comes apart in the caustic in the next one. Worse, the two hazards that matter most, chemical attack and heat, do their damage without changing how a sling looks or feels, so a sling that has lost most of its strength goes back on the rack looking exactly like one that has not.

This card is written from the negative side, family by family: not what each one is good at, but what it cannot survive.

Two rules before the table

Chemical exposure is an inhalation and contact hazard for the rigger before it is a durability question for the sling. Acid mist, caustic aerosol and solvent vapour in a work area are controlled by that facility's programme, and the safety data sheet for the substance names the route and the control. A glove does not address an airborne route; respiratory protection, where the assessment calls for it, sits under a written programme per 29 CFR 1910.134. Read the sheet before you plan the path the load takes, not after.

Never weld, heat, flame-cut or bend a chain sling to repair or shorten it. Doing so destroys the heat treatment that gives the chain its rating, so there is no arithmetic left to correct. Cutting or welding plated or coated chain also releases metal fume, which needs local exhaust or respiratory protection under a 1910.134 programme rather than a face shield, and none of it restores a rated capacity. Shortening is done with the chain manufacturer's own rated shortening component or not at all.

What each family cannot tolerate

The two synthetic rows are the ones that trip crews, because nylon and polyester are near mirror images on chemistry. Nylon holds up to alkalis and is attacked by acids; polyester holds up to most mineral acids and is attacked by alkalis and caustics. A shop that keeps one kind of web sling and uses it everywhere is right in half its bays and wrong in the other half, and there is no way to tell which sling has been in which bay by looking at it. Colour-coding by polymer and keeping the two stocks physically apart is the only control that survives a busy day.

Two more family-level statements worth stating with their exclusions attached. Aramid resists heat far better than nylon or polyester and is poor on ultraviolet and poor on abrasion, so it solves the hazard it was picked for and fails to one nobody was watching. Polypropylene tolerates a broad range of both acids and alkalis and has a lower melting point than nylon or polyester along with weak ultraviolet resistance. In every one of these cases the authority is the sling manufacturer's chemical compatibility statement for the specific product, in writing, and not a remembered family rule including the ones in this table.

Chemical damage is not a percentage you deduct

This is the part that changes what you do. Angle, hitch and bend are corrections: you measure something, look up a factor, and re-base the rating. Chemical and heat damage are not corrections, because you cannot measure the remaining strength of a sling in the field and no table converts an exposure into a fraction.

So work out what the exposure would have to be worth before it matters, and the answer is unsettling. A sling's rating is its minimum breaking force divided by a design factor, commonly 5 to 1 for wire rope, web and roundslings and 4 to 1 for alloy steel chain, per the manufacturer's rating and ASME B30.9 in the edition your jurisdiction, contract or employer programme has adopted. At 5 to 1, a sling degraded to 40 percent of its original breaking force is working at 5 x 0.40 = 2.0 to 1 when loaded to its rated capacity. Degraded to 20 percent, it is at 1.0 to 1, which means it parts at exactly the load the tag says it can hold.

Nothing about a sling tells you where in that range it is. That is why the rule for chemical and heat damage is binary rather than proportional: evidence of chemical attack or heat damage removes the sling from service, and it is removed by the person who finds it rather than set aside for a second opinion. A sling put back on the rack "to look at later" is a sling that gets used later.

Worked example: two hazards at the same time

A section of ventilation duct, 1,200 lb from the fabricator's drawing, has to be lifted and carried across an aisle in a metal finishing area. The travel path passes over a pickling line that puts acid mist in the air, and the duct section itself is coming off a bake area, so its surface is well above ambient. Two-leg bridle, hook over the centre of gravity, legs equal, measured at 60 degrees from horizontal.

First the arithmetic, which is complete and which answers only the question of size:

• Load weight, from the drawing: 1,200 lb.
• Legs counted: 2.
• Vertical share per leg: 1,200 / 2 = 600 lb.
• Angle factor, from horizontal: sine 60 degrees = 0.866. Leg tension = 600 / 0.866 = 693 lb.
• Hitch: vertical, factor 1.00.
• Bend severity: shackle pins meet the sling manufacturer's stated minimum, factor 1.00.
• Required per-leg rated capacity: 693 lb, rounded up to the next published size.

Now the selection, which the 693 lb has no opinion about:

• Nylon web: out. Acid mist is the exposure nylon is worst at, and the damage is not visible.
• Polyester web: passes the acid gate, then meets the heat gate. Its usable ceiling is the sling manufacturer's stated maximum operating temperature for that product, which is on the tag or the datasheet. Whether the duct's surface is under that ceiling is a measurement, taken with a surface thermometer from outside the load's footprint before the sling goes on, not a judgement made by touching it.
• Wire rope: not eliminated by a rating. Eliminated by inspectability. Acid attacks the wires from the inside of the rope where nobody can see it, and a wire rope sling's whole advantage is that it warns you before it fails. Take the warning away and you have the disadvantages with none of the benefit.
• Alloy steel chain: handles the heat up to the chain manufacturer's stated limit, and carries hydrogen embrittlement risk in an acid environment. Embrittlement is the worst of all the failure modes here because it gives no warning of any kind.

Every common family is eliminated or conditional. That is not a hard selection problem, it is a signal that the lift plan is wrong. Two intolerances stacked on one path means the answer is to change the path or the exposure rather than to keep shopping for a sling: move the pick outside the mist envelope, wait for the section to come down to a surface temperature under the sling's stated limit, or use a below-the-hook lifting device with rated hardware that does not have a textile in the load path. If none of those is possible, the question goes to the sling manufacturer for a written compatibility statement and to the qualified person your employer designated, not to whoever is holding the tape.

Getting it wrong here has a specific shape. Nobody drops this duct on the first pick. The polyester sling that made twelve trips through acid mist and the nylon sling that made twelve trips through a caustic wash both come back looking used rather than damaged, go on the rack, and get pulled for a heavier load six weeks later by someone who read a tag that is still perfectly legible and still perfectly wrong about that sling.

What flips the answer

• The exposure is incidental rather than continuous. A single pass through a vapour is a different question from a sling stored in that bay, and the manufacturer's compatibility statement is where that distinction gets resolved rather than in the field.
• The load has sharp edges as well as a chemical exposure. Then abrasion resistance enters the selection alongside chemistry, and corner protection has to be compatible with the same chemistry, which is the step crews skip.
• The sling will be left rigged and loaded for a long period, for example during a fit-up. Sustained load raises creep as a concern for high-modulus polyethylene specifically, and a hoist brake is not a parking device for any family, so the load gets landed and blocked rather than hung.
• The tag is missing or illegible. Then the family is unknown as well as the rating, and the sling is out of service under 29 CFR 1910.184 for general industry or 29 CFR 1926.251 for construction.

How to verify you got this right

Ask what the sling has been exposed to, not just what it will be exposed to. The history is the part no inspection recovers, and the only place it lives is in how you store and segregate stock. If nylon and polyester slings share a rack in a plant that runs both acids and caustics, your selection is a coin toss no matter how carefully you did it today.

Then check the two temperature figures separately. The sling manufacturer's maximum operating temperature is one number; the load's actual surface temperature is another, and it is measured rather than estimated. A rigger who touches the duct to judge it has verified only that it is not hot enough to burn a hand, which is far below where a synthetic sling starts losing strength.

Last, confirm your removal decision was binary. If you found evidence of chemical or heat damage and then reasoned about how much capacity might be left, you have used a method this card says does not exist. Tag it out and get it off the rack.

References

• 29 CFR 1910.184, slings, for general industry, including the removal-from-service criteria for each sling type, and 29 CFR 1926.251, rigging equipment for material handling, for construction.
• 29 CFR 1910.134, respiratory protection, where an exposure assessment calls for respiratory control in the area a lift passes through.
• ASME B30.9, slings, in the edition your authority having jurisdiction, contract or employer programme has adopted.
• The sling manufacturer's chemical compatibility statement, maximum operating temperature and identification tag for the specific product, and the safety data sheet for any substance in the work area.
• See related: How to Inspect a Synthetic Sling and What Removes It From Service; How a Wire Rope Sling Fails and What You See First.